Cloud App Security Brokers (CASB) term was coined by Gartner and it just means that something that will help you provide visibility, control of data travel, and also identify if there are any issues and combat those threats across all your cloud services.
Gartner defines Cloud Access Security Broker as something like a security enforcement that is placed in between the cloud service consumers and the cloud service providers. This is done in order to enforce the security policies for the users who are accessing the cloud-based resources. In other words, cloud access security broker is an intermediary between your users and all the cloud services that they access. The CASB helps you apply monitoring and security controls over your users and data. Think of it like a firewall in your corporate network. "